Many device manufacturing processes involve cleaning, preparation and depositing thin layers on a substrate. The degree of perfection of a thin film on a substrate is important in many and diverse applications. Examples are semiconductor layers, epitaxial, polycrystalline or amorphous, insulating layers, thin film coatings on optical components and on optical or magnetic memory devices, etching of surfaces, and preparation of non-specular surfaces e.g. solar converters.
Some of the factors determining the quality of a thin film are the perfection and the cleanliness of the substrate surface. The same is true for deposition of subsequent films. It is not easy to clean say a layer of silicon oxide off a silicon substrate without either incompletely cleaning the surface or removing material from the substrate surface. Both may degrade the interface between substrate and grown layers.
Various systems are now in use for growing layers. For example chemical vapour deposition (CVD), and molecular beam epitaxy (MBE). These processes are carried out in closed chambers that can be thoroughly cleaned. There remains the problem of knowing what is happening at the surface of a substrate in the closed chamber.
Use of scattered light to monitor growing interfaces is known from O. N. Mesquita et al, Physical Review B, Vol. 29 No. 5, 1 Mar. 1984, pages 2846-2849; and H. Durig et al, Physical Review A, Vol. 30 No. 2, Aug. 1984, pages 946-959. These describe directing light through a crystal to the crystal-liquid interface in a zone refining cylinder. Reflected light provides information about crystalline growth at the interface.
Such prior art provides no information on the cleaning and subsequent growth of epitaxial layers on a flat slice of semiconductor material.
Surface statistics, roughness, etc., can be calculated from scattered light. This is described in Optical Engineering, Jul./Aug. 1984, Vol. 23 No. 4, J. C. Stover et al, pages 406-412; Applied Optics, 15 Oct. 1984, Vol. 23 No. 20, P. Roche & E. Pelletier, pages 3561-3566; S.P.I.E. Vol. 511, Stray Radiation IV 1984, R. M. Silva et al, Pages 38-43.
Specularly reflected light has been used in the growth of coating layers. See for example U.S. Pat. No. 3,892,490, G.B. Patent No. 731,865, European Patent No. A.2, 0,150,945.
The above problem is solved according to this invention by directing a beam of light onto the surface of a substrate and detecting light scattered off the substrate in a non-specular reflection direction. Changes in the intensity of detected light are then used to change process parameters, e.g. change from a cleaning step to a layer growth step.
According to this invention a method of monitoring surface conditions on a surface of a substrate being processed includes the steps of:
directing a beam of light onto at least one small area of the surface being monitored; PA1 detecting light scattered from said small area in at least one non-specular reflection direction; PA1 changing the process parameters in response to detected changes on the surface. PA1 a closed vessel capable of holding a substrate to be processed, PA1 means for cleaning or otherwise preparing a surface of the substrate, PA1 means for depositing a layer of material on the cleaned surface, PA1 means for directing light onto at least one small area of the surface less than the total area of the surface, PA1 means for detecting light scattered from the small area in at least one non-specular reflection direction, PA1 means for changing the cleaning or layer growth parameters in response to detected changes or scattered light.
The processing of the surface may be a cleaning, a removal of oxide or other compound, a deposition of layers of materials the same or different from the original, substrate, surface. These layers may be deposited by chemical vapour deposition (CVD), metal organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE), etc.
The change in process parameters may be a change from a cleaning step to a growth of layers, or a change from growing layers of different materials or cessation of growth at a specified end point.